Fixed exception when parsing XML with comments.

Fixed exception when parsing XML with comments
Type check for comment is kind of ugly. It's `isisntance(xml_child, etree._Comment)`, so it's kind of internal class to  etree implementation. I thought it would be better to do duck-typing style check for this case.

Having said that, do you still think we should go about explicit check for the comment? Compromise approach would be to check for `tag` being `str` and log something before returning.